rds: refreshing VPC context generates inconsistent ordering that causes unnecessary DBProxy resource replacement

### Describe the bug

We do a subnet selection to get our subnets. The behaviour is non-deterministic. Cfn is too stupid to treat the list as a set, so... we get stuff like this. As you can see below it wants to replace our proxy but can't because it's a named resource. The reason it wants to replace the resource is that the order of the subnets in the list changed.

```
[~] AWS::RDS::DBProxy GlobalStagingUsWest2/DochistoryAuroraStg/Or1DochistoryStg/Proxy Or1DochistoryStgProxy87EB58B0 replace
└─ [~] VpcSubnetIds (requires replacement)
└─ @@ -4,9 +4,9 @@
[ ] "subnet-07260952ef7787a6d",
[ ] "subnet-0b868f96519464bdb",
[ ] "subnet-0d9a398b9a31f6fcc",
+ [+] "subnet-09a4826d4eb84d03c",
[ ] "subnet-0c8fd644d2a2f91c0",
[ ] "subnet-0656782790830be94",
- [-] "subnet-08a104b7791c32f08",
- [-] "subnet-09a4826d4eb84d03c",
- [-] "subnet-029c625dae0eb4783"
+ [+] "subnet-029c625dae0eb4783",
+ [+] "subnet-08a104b7791c32f08"
[ ] ]
```

As a result, the stack update is broken and we can't roll forward actual changes.

### Expected Behavior

Cfn shouldn't differentiate between sets and lists and not treat sets like the order matters.

CDK should compensate for Cfn's incompetence by always providing sets as ordered lists. Unfortunately this is probably a breaking change. But you could put it behind a feature flag, I guess.

### Current Behavior

No idea what caused the order of the subnets to change.

### Reproduction Steps

No idea what caused the order of the subnets to change.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.78.0

### Framework Version

_No response_

### Node.js Version

16.20

### OS

Mac

### Language

Typescript

### Language Version

4.9.5

### Other information

_No response_

The issue names no files or tests; begin with the subnet selection feeding AWS::RDS::DBProxy and its VpcSubnetIds, using the reported CDK CLI version as context. Compare synthesized subnet ordering across repeated runs and define done as stable ordering that avoids unnecessary replacement, with a regression test if the project provides one.

Written by the indexing model from the issue text.